Led illumination assembly with compliant foil construction

ABSTRACT

An illumination assembly includes a compliant substrate comprising a first and second electrically conductive foil separated by an electrically insulating layer. The insulating layer includes a polymer material loaded with particles that enhance thermal conductivity of the insulating layer. A plurality of LED dies are disposed on the first conductive foil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 11/669,622, filed Jan.31, 2007, now allowed, the disclosure of which is incorporated byreference in its entirety herein.

FIELD OF THE INVENTION

The present invention relates to light emitting diode (LED) devices,liquid crystal display (LCD) devices, components therefor, and relatedarticles and processes.

BACKGROUND

LEDs are a desirable choice of light source in part because of theirrelatively small size, low power/current requirements, rapid responsetime, long life, robust packaging, variety of available outputwavelengths, and compatibility with modern circuit construction. Thesecharacteristics may help explain their widespread use over the past fewdecades in a multitude of different end use applications. Improvementsto LEDs continue to be made in the areas of efficiency, brightness, andoutput wavelength, further enlarging the scope of potential end-useapplications.

Recently, LEDs have begun to be used for backlighting purposes in LCDtelevision devices, as well as other types of lighting and displaysystems. For most lighting applications, it is necessary to have aplurality of LEDs to supply the required light intensity. Because oftheir relatively small size, a plurality of LEDs can be assembled inarrays having small dimensions and a high luminance or irradiance.

It is possible to achieve an increase in the light density of an arrayof LEDs by increasing the packing density of the individual LEDs withinthe array. An increase in packing density can be achieved by increasingthe number of LEDs within the array without increasing the spaceoccupied by the array, or by maintaining the number of LEDs within thearray and decreasing the array dimensions. However, tightly packinglarge numbers of LEDs in an array is a long term reliability concernsince local heating, even with a globally efficient thermal conductionmechanism, can reduce the lifespan of the LEDs. Therefore, dissipatingthe heat generated by the array of LEDs becomes more important as thepacking density of the LEDs increases. In other applications, even thosewithout high packing densities, the driving voltages/currents andbrightness of LED dies are increasing, leading to increases in localtemperatures around the LED dies. Consequently, there is a need forbetter heat dissipation at the location of each LED die, as well asacross the array.

Conventional LED mounting techniques use packages like that illustratedin U.S. Patent Application Publication 2001/0001207A1 (Shimizu et al.),that are unable to quickly transport the heat generated in the LED awayfrom the LED. As a consequence, performance of the device is limited.More recently, thermally enhanced packages have become available, inwhich LEDs are mounted and wired on electrically insulating butthermally conductive substrates such as ceramics, or with arrays ofthermally conductive vias (e.g., U.S. Patent Application Publication2003/0001488A1 (Sundahl)), or use a lead frame to electrically contact adie attached to a thermally conductive and electrically conductivethermal transport medium (e.g., U.S. Patent Application Publication2002/0113244A1 (Barnett et al.)). An illumination assembly havingimproved thermal properties is disclosed in U.S. Patent ApplicationPublication 2005/0116235A1 (Schultz et al.), in which an illuminationassembly includes a plurality of LED dies disposed on a substrate havingan electrically insulative layer on a first side of the substrate and anelectrically conductive layer on a second side of the substrate. EachLED die is disposed in a via extending through the electricallyinsulative layer on the first side of the substrate to the electricallyconductive layer on the second side of the substrate, and each LED dieis thermally and electrically connected through the via to theelectrically conductive layer. The electrically conductive layer ispatterned to define a plurality of electrically isolated heat spreadingelements which are in turn disposed adjacent a heat dissipationassembly.

Applicants of the present application have found that, although the morerecent approaches improve the thermal properties of LED arrays, thereare disadvantages to these approaches. Specifically, the substrates onwhich the LED arrays are disposed have limited ability to form localfeatures having sizes useful for fully using, controlling, andmanipulating the light emitted from the LEDs.

BRIEF SUMMARY

The present application discloses, inter alia, illumination assembliesthat include a compliant substrate having a first and secondelectrically conductive foil separated by an electrically insulatinglayer. The insulating layer includes a polymer material loaded withparticles that enhance thermal conductivity of the insulating layer. Aplurality of LED dies are preferably disposed on the first conductivefoil.

In exemplary embodiments, the compliant substrate has at least onedeformation, and at least one of the LED dies is disposed on or in thedeformation. In some embodiments, the first and second electricallyconductive foils and the electrically insulating layer are altered tocontrol the optical properties of the substrate.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective illustration of a portion of an illuminationassembly;

FIG. 2 is a top plan view of a portion of the illumination assembly ofFIG. 1, showing a larger surface area of the illumination assembly;

FIG. 3 is an enlarged cross-sectional illustration taken along line 3-3of FIG. 2;

FIG. 4 is an enlarged cross-sectional illustration showing anotherillumination assembly;

FIG. 5A is a perspective illustration of an illumination assembly havinga plurality of inwardly projecting deformations having LEDs disposedtherein;

FIG. 5B is a perspective illustration of an illumination assembly havingan outwardly projecting deformation having LEDs disposed thereon;

FIG. 6A is a cross-sectional illustration of an illumination assemblywith a compliant substrate having an inwardly projecting deformationwith an LED disposed therein, wherein the substrate is conformablyattached to a substrate;

FIG. 6B is another cross-sectional illustration of an illuminationassembly with a compliant substrate having an inwardly projectingdeformation with an LED disposed therein, wherein a thermal interfacematerial conforms to the deformed substrate;

FIG. 6C is a cross-sectional illustration of an illumination assemblywith a compliant substrate similar to FIG. 6A, showing optional use withan encapsulant and optical film;

FIG. 7 is a cross-sectional illustration of an illumination assemblywith a compliant substrate having an outwardly projecting deformationwith LEDs disposed thereon, wherein a thermal interface materialconforms to the deformed substrate; and

FIG. 8 is a schematic illustration of one method of making anillumination assembly.

In the Figures, like reference numerals designate like elements. TheFigures are idealized, not drawn to scale, and intended for illustrativepurposes only.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

We describe herein illumination assemblies that include LED dies. Inthis regard, “light emitting diode” or “LED” refers to a diode thatemits light, whether visible, ultraviolet, or infrared. It includesincoherent encased or encapsulated semiconductor devices marketed as“LEDs”, whether of the conventional or super radiant variety. If the LEDemits non-visible light such as ultraviolet light, and in some caseswhere it emits visible light, it can be packaged to include an organicor inorganic phosphor (or it may illuminate a remotely disposedphosphor) to convert short wavelength light to longer wavelength visiblelight, in some cases yielding a device that emits white light. An “LEDdie” is an LED in its most basic form, i.e., in the form of anindividual component or chip made by semiconductor processingprocedures. For example, the LED die is ordinarily formed from acombination of one or more Group III elements and of one or more Group Velements (III-V semiconductor). Examples of suitable III-V semiconductormaterials include nitrides, such as gallium nitride, and phosphides,such as indium gallium phosphide. Other types of III-V materials can beused also, as might inorganic materials from other groups of theperiodic table. The component or chip can include electrical contactssuitable for application of power to energize the device. Examplesinclude wire bonding, tape automated bonding (TAB), or flip-chipbonding. The individual layers and other functional elements of thecomponent or chip are typically formed on the wafer scale, and thefinished wafer can then be diced into individual piece parts to yield amultiplicity of LED dies. The LED die may be configured for surfacemount, chip-on-board, or other known mounting configurations. Somepackaged LEDs are made by forming a polymer encapsulant formed over anLED die and an associated reflector cup.

As described further below, the LED dies can be disposed on a compliantsubstrate. In this regard, a foil, a substrate, or other thin article isreferred to as “compliant” if localized force or pressure can be used topermanently deform the article without substantial cracking or loss offunctionality. The deformation, which may be a protrusion or adepression, can be isolated to only a portion of the article, such thatif the article is laid flat, the deformation is bounded on all sides byflat portions of the article. Stated differently, the deformation canhave a compound curvature, i.e., can be curved (whether smoothlyvarying, as in the case of a hemisphere, or piecewise discontinuous, asin the case of a pyramidal shape with flat facets) in each of twomutually perpendicular reference planes, the reference planes beingperpendicular to the plane of the article. Preferably, the permanentdeformation can be produced with moderate pressures, such as thoseachieved by pressing a small blunt object against the article by hand.

Turning now to FIG. 1, a perspective view of a portion of anillumination assembly 10 is illustrated. The illumination assembly 10includes a plurality of LED dies 20 disposed in an array on a compliantsubstrate 30. The LED dies 20 can be selected to emit a preferredwavelength, such as in the red, green, blue, ultraviolet or infraredspectral regions. The LED dies 20 can each emit in the same spectralregion, or in different spectral regions. In some cases, the LED dies 20are nominally 250 μm tall.

The compliant substrate 30 includes a first electrically conductivelayer 32 defining a top surface 34 of the substrate, and a secondelectrically conductive layer 36 defining a bottom surface 38 of thesubstrate 30. The first and second electrically conductive layers 32, 36are separated by an electrically insulating layer 40. As illustrated,the first electrically conductive layer 32 is patterned to formelectrical circuit traces 41, and the LED dies 20 are disposed on andelectrically connected to the first conductive layer 36. The illustratedcircuit traces 41 are exemplary only.

The second electrically conductive layer 36 of substrate 30 is disposedadjacent a heat sink or heat dissipation assembly 50, and is thermallycoupled thereto by a layer 52 of thermal interface material. The heatdissipation assembly 50 can be, for example, a heat dissipation device,commonly called a heat sink, made of a thermally conductive metal suchas aluminum or copper, or a thermally conductive polymer such as acarbon-filled polymer. The layer 52 of thermal interface material maycomprise any suitable material, including adhesives, greases, andsolder. The thermal interface material of layer 52 may be, for example,a thermally conductive adhesive material such as a boron nitride loadedpolymer (e.g., 3M™ Thermally Conductive Tape 8810 sold by 3M Company),or a thermally conductive non-adhesive material such as a silver filledcompound (e.g., Arctic Silver™ 5 High-Density Polysynthetic SilverThermal Compound sold by Arctic Silver Incorporated of Visalia, Calif.,U.S.A.). Preferably, heat dissipation assembly 50 has a thermalimpedance as small as possible, preferably less than 1.0° C./W. In somecases, heat dissipation assembly 50 preferably has a thermal impedancein the range of 0.5 to 4.0° C./W. The material of layer 52 desirably hasa thermal conductivity in the range of 0.1 W/m-K to 10 W/m-K, preferablyat least 1 W/m-K.

In the illumination assembly 10 of FIG. 1, the LED dies 20 are of thetype having electrical contacts on opposed sides of the LED die,referred to as the base and top surface of the die. The contact on thebase of each LED die 20 is electrically and thermally connected to acircuit trace 41 immediately beneath the LED die 20. The contact on thetop of each LED die 20 is electrically connected to another circuittrace 41 by a wirebond 39 extending from LED die 20. To facilitate goodwirebonding, first conductive layer 32 can include a surfacemetallization of nickel and gold.

The pattern of first conductive layer 32 of FIG. 1 is best seen in FIG.2. First conductive layer 32 is patterned to define a plurality ofcircuit traces 41. Each circuit trace 41 is positioned for electricaland thermal coupling to an associated LED die 20 and also to anassociated wirebond 39, such that at least some LED dies 20 areelectrically connected in series, based on requirements of theparticular application. As best seen in FIG. 2, instead of patterningfirst conductive layer 32 to provide only narrow conductive wiringtraces to electrically connect the LED dies 20, the first conductivelayer 32 can be patterned to remove only as much conductive material asis necessary to electrically isolate the circuit traces 41, leaving asmuch of first conductive layer 32 as possible to act as a reflector forthe light emitted by LED dies 20. Leaving as much of first conductivelayer 32 as possible also results in wider circuit traces that areuseful for applications requiring short high current pulses. The widertraces allow a higher current density to be delivered, even over veryshort times.

In some embodiments, the material of first conductive layer 32 isselected to provide the desired optical properties (e.g., reflectance,color, scattering, diffraction, or a combination of these properties)for the particular application. In other embodiments, the opticalproperties of top surface 34 of first conductive layer 32 are enhancedby plating and/or coating to provide the desired optical properties. Insome embodiments, top surface 34 is plated, and then the exposed surfaceof the plating is coated to improve the optical performance. Suitablecoating and plating materials include silver, passivated silver, gold,rhodium, aluminum, enhanced reflectivity aluminum, copper, indium,nickel (e.g., immersion, electroless or electroplated nickel), chromium,tin, and alloys thereof. In some embodiments, a coating may comprise awhite coating such as a highly reflective white polymer, e.g., StarbriteEF reflective coatings sold by Spraylat Corporation, Pelham, N.Y.Multilayer dielectric stacks can also be deposited on the surface 34 oflayer 32 for enhanced reflectivity. Suitable coatings may also includemetal and semiconductor oxides, carbides, nitrides, as well as mixturesand compounds thereof. These coatings may be electrically conductive orinsulating depending upon the intended application. Suitable coatingmethods include sputtering, physical vapor deposition, and chemicalvapor deposition. The coating process may optionally be ion assisted.The optical properties of the conductive layer 32 and platings orcoatings thereon can also be modified by controlling the surface textureof the surface 34 and/or the platings and coatings described previously.For example an optically smooth surface finish may be preferred in somecases, a matte or somewhat roughened surface finish in other cases. Inother embodiments, optical films, such as Vikuiti™ Enhanced SpecularReflectivity (ESR) film sold by 3M Company, may also be applied to oneor both major surfaces of the first layer 32 to increase desirableoptical properties, e.g., specular or diffuse reflectivity.

As shown in FIG. 3, the electrically insulating layer 40 of substrate 30comprises a polymer material 43 loaded with particles 42 that enhancethermal conductivity of the insulating layer 40. The polymer material 43and/or particles 42 can also be selected to alter the electrical,thermal, optical and/or mechanical properties of the insulating layer40. When the electrical design includes large areas of exposedelectrically insulating layer 40 near the LEDs, the optical properties(e.g., reflectivity, diffusivity, transparency) of the electricallyinsulating layer 40 can also be enhanced.

As mentioned above, the polymer material 43 and/or particles 42 can beselected to enhance the reflectivity of the insulating layer 40. Forexample, insulating layer 40 can be loaded with white, diffuselyreflective materials e.g., BaSO₄, TiO₂, or with high refractive indexmaterials, e.g., diamond, SiC, Al₂O₃, or with reflective materials,e.g., silver flakes or nanoparticle materials or materials oriented withelectrical/magnetic means for desired optical properties such asferroelectrics, e.g., PLZT. Alternatively, the polymer material 43and/or particles 42 can be selected to cause the insulating layer 40 tobe substantially transparent. In this case the optical properties of thecoated side of the second electrically conductive layer 36 may beselected or altered to provide desired characteristics (e.g.,reflectivity, diffusivity). In other embodiments, the polymer material43 and/or particles 42 are selected to cause the insulating layer 40 tohave a desired apparent color.

In each of these embodiments, an encapsulant may be provided on each LEDdie 20 to help couple light out of the die, and/or to preferentiallydirect the emitted light towards the insulating layer 40 to be reflected(whether specularly or diffusely), polarized, or waveguided by theinsulating layer 40. The macro-, micro-, and nanostructure of theinsulating layer 40 can be engineered for specific optical properties bypre-forming the inner major surfaces of the metal foils (i.e., theinterface of electrically insulating layer 40 with first electricallyconductive layer 32 and with second electrically conductive layer 36.For example, the inner surface of a copper foil can be structured bychemical (grain etching), mechanical (embossing), or optical (laserablation) means. The exposed insulating layer 40 interface will be theinverse or mirror image of the metal film pre-form. The opticalproperties of the insulating layer 40 can also be modified by theaddition of one or more phosphor or fluorescent materials into theinsulating layer 40 so that a shift in the wavelength of the incidentradiation occurs. Efficient removal of the Stokes shift energy in thesecases of wavelength conversion is an additional benefit.

In some cases, the electrically insulating layer 40 is prepared from ablend of resin and particles. Suitable resins include epoxies and blendsthereof. Commercially available epoxies include Epon™ 1001F epoxy resinsold by Resolution Performance Products, and XP71756 epoxy sold byVantico Inc. The resin can withstand temperatures that would beencountered in a typical solder reflow operation, for example, in therange of about 180 to about 290° C. Preferably, the resin can withstandshort term exposure to temperatures over 300° C. needed to reflow 80/20gold/tin solder commonly used for LED die attachment. These resins maybe dried or cured to form the electrically insulating layer.

The particles 42 are preferably selected to enhance the thermalconductivity of the insulating layer 40. Any suitable materials can bechosen for this purpose. In exemplary embodiments, the particles arecomposed of silicon carbide, aluminum oxide, boron nitride, diamond, ormore complex, engineered materials such as metallic particles withelectrically insulating coatings or nanoparticles. The particles can bedielectric (electrically insulating) or electrically conductive ormixtures thereof, provided that the overall effect of the blend of resinand particles is electrically insulative with adequate thermalconductivity for the intended application.

Exemplary dielectric or electrically insulating particles include bariumtitanate, barium strontium titanate, titanium oxide, lead zirconiumtitanate, boron, boron nitride, diamond, alumina, beryllium, silicon, aswell as other carbides, oxides, and nitrides of those materials, andcompounds or mixtures thereof. A commercially available barium titanateis available from Nippon Chemical Industrial Co., Tokyo, Japan, underthe trade designation “BESPA AKBT.”

Exemplary electrically conductive particles may comprise electricallyconductive or semiconductive materials such as metal or metal alloyparticles, where the metal may be silver, nickel, or gold; nickel-coatedpolymer spheres; gold-coated polymer spheres (commercially availablefrom JCI USA Inc., New York, N.Y., under product designation number “20GNR4.6-EH”); or mixtures thereof.

The particles may be any shape and may be regularly or irregularlyshaped. Exemplary shapes include spheres, platelets, cubes, needles,oblate, spheroids, pyramids, prisms, flakes, rods, plates, fibers,chips, whiskers, and mixtures thereof. The particle size, i.e., thesmallest dimension of the particle, typically ranges from about 0.05 toabout 11 μm, preferably 0.05 to 3 μm, more preferably 0.05 to 2 μm.Particles can be substantially the same size, or mixtures of differentsizes of particles can be used. In order to form a sufficiently smoothinsulating layer 40 for the promotion of adhesion with first and secondelectrically conductive layers 32, 36, the average size of the particlesis desirably a fraction of the thickness of the electrically insulatinglayer 40. In some embodiments, the average size of the particles is lessthan about ½ of the thickness of the electrically insulating layer 40,preferably less than about ¼ Of the thickness of the electricallyinsulating layer 40, more preferably less than about 1/10 of thethickness of the electrically insulating layer 40.

The loading of particles in the polymer is typically 20 to 60% byvolume, based on the total volume of the electrically insulating layer.Particle distribution may be random or ordered. Loading of particles inthe polymer may be greater than 60% by volume if surfaces of the firstand second electrically conductive layers 32, 36 that adjoin insulatinglayer 40 are treated to provide improved adhesion with the insulatinglayer 40. Exemplary surface treatments that are useful in providingimproved adhesion include 5-aminobenzotriazole and3-glycidoxypropyltrimethoxysilane, corona discharge, plasmaashing/etching, self-assembled monolayers, and reactive layers to bindthe resin matrix material to the first and second electricallyconductive layers 32, 36.

Metal foils can also be treated with anti-corrosion treatments toimprove adhesion (e.g., the use of zinc/chromium treatments for copperfoil).

Typically, the thickness of the electrically insulating layer 40 rangesfrom about 0.5 to about 40 μm, preferably less than about 20 μm.

In some embodiments, the first and second electrically conductive layers32, 36 comprise an electrically conductive foil. The electricallyconductive foils are composed of a metal or conductive plastic. Suitablemetal foils include copper, aluminum, nickel, gold, silver, palladium,tin, lead, and combinations thereof, for example aluminum clad copperfoil. When the first and second electrically conductive layers are metalfoils, the metal preferably has an anneal temperature which is at orbelow the temperature for curing the electrically insulating layer, orthe metal is annealed before the electrically insulating layer iscoated.

Typically, the first and second electrically conductive foil layers havea thickness ranging from 0.5 to 8 mils (approximately 10 to 200 μm),more preferably 0.5 to 1.5 mils (approximately 10 to 38 μm).Furthermore, it is often desirable for the first and second electricallyconductive foil layers to each be thicker than the insulating layer. Insome cases, the thickness of the first conductive foil layer 32 isapproximately the same as that of the second conductive foil layer 36.In other cases, the thickness of the first conductive foil layer 32 isdifferent than that of the second conductive foil layer 36. In somecases, the thickness of the second conductive foil layer 36 is greaterthan that of the first conductive foil layer 32, such that secondconductive foil layer 36 functions to more effectively spread heatlaterally from the location of an LED die 20.

FIG. 3 is an enlarged sectional view taken along line 3-3 of FIG. 2. TheLED die 20 is positioned on the top surface 34 of first conductive layer32 and electrically connected to the circuit trace of first conductivelayer 32 with a layer 60 of either isotropically conductive adhesive(for example, Metech 6144S, available from Metech Incorporated ofElverson, Pa., U.S.A.,), or an anisotropically conductive adhesive, orsolder. Solders typically have a lower thermal resistance thanadhesives, but not all LED dies have solderable base metallization.Solder attachment can also have the advantage of LED die 20self-alignment, due to the surface tension of the molten solder duringprocessing. Some LEDs may be supplied with a high temperature 80/20gold/tin solder which can be reflowed to form a very stable, low thermalresistance interface capable of withstanding subsequent solderingprocesses up to 300° C. However, some LED dies 20 may be sensitive tosolder reflow temperatures, making an adhesive preferable in layer 60.

Referring now to FIG. 4, a cross-sectional illustration of anotherillumination assembly shows an LED die 20′ having both electricalcontact pads on the same side of the LED die, rather than on oppositesides of the diode as in the wirebonded embodiments of FIGS. 1-3.Depending upon the design of the LED die 20′, light is emitted from theside of the diode 20′ that is opposite the contact pads, or from theside of the diode 20′ that is on the same side as the contact pads. Aswith the wirebond LED dies 20 of FIGS. 1-3, electrically conductiveadhesives, anisotropically conductive adhesives, or solder re-flow areamong the attachment methods that can be used to attach the LED die 20′to the first conductive layer 32.

As described above, substrate 30 is a compliant material that can bepermanently deformed under moderate pressures to include protrusions ordepressions that are isolated to only a portion of the substrate 30,such that if the substrate 30 is laid flat, the deformations are boundedon all sides by flat portions of the substrate 30. When the substrate isdeformed, the insulating layer 40 remains intact and adherent (i.e., theinsulating layer 40 does not crack, fracture or delaminate from firstand second electrically conductive layers 32, 36). FIGS. 5A and 5Bprovide perspective views of a portion of illumination assemblies havingdeformations in the substrate 30, where LED dies 20 are disposed in oron the deformations. In both FIGS. 5A and 5B, for purposes of clarityand illustration, circuit traces and wirebonds are not shown.

In FIG. 5A, a plurality of substantially hemispherical depressions 70(i.e., dimples) extend below the top surface 34 of substrate 30, and anLED die 20 is disposed in each depression 70. The LED dies 20 areillustrated as being disposed substantially at the bottom center of thedepressions 70, such that LED dies 20 emit light in a directionsubstantially orthogonally aligned with the upper surface 34 of thesubstrate 30. In other embodiments, one, some, or all of the LED dies 20can be disposed on an inclined surface of their respective depression70, such that at least some LED dies 20 emit light obliquely withrespect to the upper surface 34 of the substrate 30.

In FIG. 5B, an elongated protrusion 80 (i.e., a ridge) extends above thetop surface 34 of substrate 30, and a plurality of LED dies 20 aredisposed on the protrusion 80. The LED dies 20 are illustrated as beingdisposed on both inclined surfaces of the protrusion, such that the LEDdies emit light in oblique directions with respect to the upper surface34 of the substrate 30. In other embodiments, LED dies can be mounted onthe uppermost portion of the protrusion 80, and LED dies 20 may bemounted on only one, or less than all inclined surfaces of theprotrusion 80.

The individual deformations of substrate 30 may be configured to receivea single LED die, die clusters, or banks or rows of LED dies. In someembodiments, more than one LED (e.g. LEDs having respective red, green,and blue color outputs) are closely positioned in a localized area, suchas on or in a single deformation, to generate apparent white light. Theshape of the deformation alone, or the shape of the deformation incombination with an optional encapsulant and/or an optical film, can beconfigured to enhance color mixing.

It is understood that the shapes and arrangements of depressions 70 andprotrusion 80 in FIGS. 5A and 5B are illustrative only, and are in noway intended to be limiting. Deformations in substrate 30 may be of anyshape or arrangement as is useful in the intended application of theillumination assembly 10, and include deformations having smoothlyvarying surfaces, as in the case of a hemisphere, or piecewisediscontinuous surfaces, as in the case of a pyramidal shape with flatfacets. The deformations may be asymmetrical or symmetrical, e.g. anelliptical depression rather than a hemispherical depression. In someembodiments, the deformations have compound curvature. In someembodiments, the deformations have lateral dimensions on the same orderof magnitude as lateral dimensions of the LED dies 20.

Referring now to FIGS. 6A through 7, exemplary cross-sectionalillustrations of the compliant substrate 30 are provided in whichcompliant substrate 30 has a deformation, and at least one LED die 20 isdisposed on or in the deformation.

In FIG. 6A, the patterned compliant substrate 30 has been deformed toform a depression (such as depressions 70 of FIG. 5A) of sufficient sizeto receive LED die 20 on a bottom surface of the depression. Asdescribed above, conductive adhesives, anisotropically conductiveadhesives, or solder re-flow are among the attachment methods that canbe used to attach the LED die 20 to the first conductive layer 32. Inthe illustrated embodiment, heat dissipation assembly 50 has beenpreformed with the desired depression, and substrate 30 is conformablyattached to heat dissipation assembly 50 by layer 52 of thermalinterface material having relatively constant thickness betweensubstrate 30 and heat dissipation assembly 50. An optional encapsulant90 is illustrated covering LED die 20.

FIG. 6B shows a portion of an illumination assembly similar to that ofFIG. 6A, but wherein the heat dissipation assembly 50 has asubstantially flat surface to which substrate 30 is attached. The layer52 of thermal interface material is displaced by the depression 70 andconforms to the shape of substrate 30. The reduced thickness of layer 52reduces the thermal impedance from the layer of thermal interfacematerial.

FIG. 6C also shows a portion of an illumination assembly similar to thatof FIG. 6A, but wherein the depression 30 has a depth greater than theheight of LED die 20. In the embodiment of FIG. 6C, an optionalencapsulant 90 is shown filling the depression 70 substantially flushwith the top surface of the substrate 30, and one or more optionaloptical film(s) 92, such as a diffusing film, a polarizing film (such asany of the Vikuiti™ DBEF films available from 3M Company), or astructured surface film (such as any of the Vikuiti™ BEF films availablefrom 3M Company), are used in combination with the assembly. In otherembodiments, depression 70 may have no encapsulant 90, or be less thenfull of encapsulant 90.

Referring now to FIG. 7, the patterned compliant substrate 30 has beendeformed to form a protrusion 80. As described above, conductiveadhesives, anisotropically conductive adhesives, or solder re-flow areamong the attachment methods that can be used to attach the LED dies 20to the first conductive layer 32. Heat dissipation assembly 50 has asubstantially flat surface to which substrate 30 is attached, and thelayer 52 of thermal interface material conforms to the deformed shape ofsubstrate 30. In other embodiments, the heat dissipation assembly can bepreformed with the desired shape of protrusion 80, and substrate 30 canbe conformably attached to heat dissipation assembly 50 by layer 52 ofthermal interface material.

The exemplary embodiments described herein are particularly useful whenused in combination with known encapsulants and/or known optical films.For example, encapsulants having a phosphor layer (for color conversion)or otherwise containing a phosphor can be used on or around the LED die20 without degrading the LED die light output. Encapsulants can be usedin conjunction with deformations in substrate 30 having any shape orconfiguration, including deformations extending below the upper surface34 of the substrate 30, and deformations protruding above the uppersurface 34 of the substrate 30.

Referring now to FIG. 8, in making an illumination assembly 10, thecompliant substrate 30 as described above is provided, such as byunwinding a supply roll 100 of the compliant substrate, and the firstelectrically conductive layer 32 is patterned at patterning station 102to form the desired circuit traces 41. Patterning of layer 32 may beaccomplished using any traditional circuit construction technique. LEDdies 20 are attached to the patterned first electrically conductivelayer 32 at die attach station 104 using known and conventional dieattach and wire bonding methods as described above. The compliantsubstrate 30, having LED dies 20 thereon, is then deformed at shapingstation 106 to provide the desired surface features (i.e., depressions,protrusions or combinations there) to substrate 30, with locations ofthe surface features corresponding to locations of the LED dies 20.Next, encapsulant 90 is optionally applied at encapsulation station 108and thereafter cured before the substrate 30 with LED dies 20 thereon iswound onto take-up roll 110. In othercases, deforming of the compliantsubstrate 30 can be performed before the LED dies 20 are attached, asindicated by shaping station 106′. In some cases, instead of being woundonto take-up roll 110, the compliant substrate 30 having LED dies 20thereon is cut at intervals to provide a plurality of illuminationassembly strips, panels, or other shapes suitable for mounting in abacklight, for use e.g. in backlit displays, signs, or graphics. In stilother cases, the take-up roll 110 can become a supply roll forsubsequent processing steps.

Deforming of the substrate 30 with LED dies 20 thereon may beaccomplished using many different techniques. In one technique, one ormore blunt objects of the desired shape can be pressed by hand in thecompliant substrate to form the desired depressions or protrusions. Inanother technique, the substrate 30 with LED dies 20 thereon is embossedor stamped using tools configured to prevent damage to LED dies 20 orthe electrical interconnections thereof. Preferably, a shaped tool withone or more desired deformations is provided. The compliant substrate ispositioned relative to the tool at a sequence of one or more locationsand the tool pressed onto the compliant substrate to thereby deform thesubstrate with the desired pattern. The stamping operation can use airpressure, mechanical means, hydraulic pressure, or other methods ofstamping, embossing, or coining objects known in the art.

If desired, the substrate 30 with LED dies 20 thereon can be conformablyattached to a support surface (such as heat dissipation assembly 50)which includes the desired features. The support surface may bepartially or fully formed to the desired final form of the substrate 30prior to bonding of the substrate 30 to the support surface, or thesupport surface may be formed at the same time the substrate 30 isdeformed to create the desired surface features. Shaping or deforming ofthe substrate 30 to the support surface features may be accomplishedusing techniques including vacuum molding/pressing, or laminating withor without heat and/or pressure.

The disclosed compliant substrate can be used not only with LED dies asdiscussed above, but with other circuit components, particularlycomponents that generate substantial heat. Thus, we contemplateassemblies similar to the foregoing disclosed illumination assembliesbut wherein some or all of the LED dies are replaced by one or more of:organic light emitting diodes (OLEDs), solid state lasers, powertransistors, integrated circuits (ICs), and organic electronics.

Unless otherwise indicated, all numbers expressing quantities,measurement of properties, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the specification and claims areapproximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings of the present invention. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of the claims, each numerical parameter should at least beconstrued in light of the number of reported significant digits and byapplying ordinary rounding techniques. Notwithstanding that thenumerical ranges and parameters setting forth the broad scope of theinvention are approximations, the numerical values set forth in thespecific examples are reported as precisely as possible. Any numericalvalue, however, inherently contains certain errors necessarily resultingfrom the standard deviations found in their respective testingmeasurements.

The foregoing description is illustrative and is not intended to limitthe scope of the invention. Variations and modifications of theembodiments disclosed herein are possible, and practical alternatives toand equivalents of the various elements of the embodiments would beunderstood to those of ordinary skill in the art upon study of thispatent document. These and other variations and modifications of theembodiments disclosed herein may be made without departing from thescope and spirit of the invention.

1. An illumination assembly, comprising: a compliant substratecomprising a first and second electrically conductive foil separated byan electrically insulating layer, the insulating layer comprising apolymer material loaded with particles that enhance thermal conductivityof the insulating layer, wherein the particles are oriented to providediffractive optical properties to the insulating layer; and a pluralityof LED dies disposed on the first conductive foil, wherein the compliantsubstrate has at least one deformation, and at least one of the LED diesis disposed on or in the deformation, and wherein the at least one LEDdie is disposed on an inclined surface of the deformation such that theLED die emits light obliquely with respect to the compliant substrate.2. An illumination assembly, comprising: a compliant substratecomprising a first and second electrically conductive foil separated byan electrically insulating layer, the insulating layer comprising apolymer material loaded with particles that enhance thermal conductivityof the insulating layer, wherein the particles are oriented to effectpolarization of light incident upon the insulating layer; and aplurality of LED dies disposed on the first conductive foil, wherein thecompliant substrate has at least one deformation, and at least one ofthe LED dies is disposed on or in the deformation, and wherein the atleast one LED die is disposed on an inclined surface of the deformationsuch that the LED die emits light obliquely with respect to thecompliant substrate.